HYDROLOGICAL CONNECTIVITY: DRIVING FORCE FOR THE DYNAMICS OF ORGANIC MATTER POOLS IN FLOODPLAINS OF THE RIVER DANUBE, AUSTRIA

Along the free-flowing section of the River Danube downstream of Vienna a part of the originally huge floodplain system still exists. Corresponding to the hydrological connectivity with the river, the floodplain is characterized by a gradient of extremely isolated to dynamic sections.
This study was focused on the dynamics of POM (including plankton) and DOM pools related to the degree of hydrological connectivity in two floodplain sections and the river in the years 1997 and 1998. Plankton compartments, detritus and DOM were quantified seasonally, as well as the concentrations of inorganic particles and nutrients. Stable isotope analyses were performed to identify potential sources of organic matter. 13C signatures of autochtonous POM, produced by phytoplankton and macrophytes in the floodplain, were more negative than riverine POM.
Quantity and composition of the organic matter pools varied with hydrological connectivity. Maximum concentrations of 7.37mg.l-1 POM were recorded during phases of high connectivity and the fraction of detrital and allochtonous POM, mainly originating from riverine and terrestrial sources, increased. With receding water levels DOM composition was influenced by enhanced phytoplankton development, POM concentrations decreased due to sedimentation. During phases of low connectivity the plankton biomass increased constituting the major part of the POM pool.